Fair Value - Partnership's Financial Assets at Fair Value (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Investment securities:
Total investment securities	$ 250	$ 258
Inventory securities:
Total inventory securities	43	50
Equities [Member]
Investment securities:
Total investment securities	2	2
Inventory securities:
Total inventory securities	16	16
Corporate Bonds and Notes [Member]
Investment securities:
Total investment securities	1	1
Inventory securities:
Total inventory securities	3	2
Government and Agency Obligations [Member]
Investment securities:
Total investment securities	3	3
U.S. Treasuries [Member]
Investments segregated under federal regulations:
Total investments segregated under federal regulations	998	2,399
Mutual Funds [Member]
Investment securities:
Total investment securities	244	252
Inventory securities:
Total inventory securities	6	4
State and Municipal Obligations [Member]
Inventory securities:
Total inventory securities	15	24
Certificates of Deposit [Member]
Cash equivalents:
Cash and cash equivalents	298	275
Inventory securities:
Total inventory securities	3	4
Level I [Member]
Investment securities:
Total investment securities	249	257
Inventory securities:
Total inventory securities	22	20
Level I [Member] | Equities [Member]
Investment securities:
Total investment securities	2	2
Inventory securities:
Total inventory securities	16	16
Level I [Member] | Government and Agency Obligations [Member]
Investment securities:
Total investment securities	3	3
Level I [Member] | U.S. Treasuries [Member]
Investments segregated under federal regulations:
Total investments segregated under federal regulations	998	2,399
Level I [Member] | Mutual Funds [Member]
Investment securities:
Total investment securities	244	252
Inventory securities:
Total inventory securities	6	4
Level II [Member]
Investment securities:
Total investment securities	1	1
Inventory securities:
Total inventory securities	21	30
Level II [Member] | Corporate Bonds and Notes [Member]
Investment securities:
Total investment securities	1	1
Inventory securities:
Total inventory securities	3	2
Level II [Member] | State and Municipal Obligations [Member]
Inventory securities:
Total inventory securities	15	24
Level II [Member] | Certificates of Deposit [Member]
Cash equivalents:
Cash and cash equivalents	298	275
Inventory securities:
Total inventory securities	$ 3	$ 4